Rotary drum reactors are not new in a broad sense. They are used, for example, in the production of granular phosphate fertilizers. In these reactors phosphoric acid and ammonia are injected simultaneously into a tumbling or rolling bed of granular fertilizer within the drum, and reaction between the acid and the ammonia occurs within the bed.
The present invention provides a rotary drum reactor which effects sequential reaction of the material with different reactants. The reactants are typically acid and ammonia and these terms are used throughout this description. The construction and operation of the drum reactor is not, however, restricted to the use of these two reactants. The manner of contacting the particulate material with the acid and with the ammonia permits close control of the reactions to obtain a product of desired composition. This feature has general utility in various reactions and special utility in the conversion of poultry litter into a feed supplement for ruminant animals. By sequencing the acidifcation and ammoniation steps, it is possible to control the acid reaction independently of the ammoniation. The acid reaction may be only an impregnation of the particulate material with the acid or it may include various chemical and physical reactions between the acid and the material. In either case the special manner of contacting the acid with the material assures rapid and uniform contact between the two. This is of particular importance in reactions, such as the acidification of poultry litter, where it is desired to rapidly reduce the pH of the particulate material to a predetermined level. The ammonia is then introduced into the acidified mass in a manner to react therewith rapidly so that no or essentially no free ammonia is present in the atmosphere within the drum.
In one particularly useful construction the means for introducing the ammonia is longitudinally spaced in a downstream direction (with respect to the direction of travel of the particulate material) in order to provide an intermediate chamber for extending the reaction time between the acid and the material as the latter travels through the intermediate chamber. It is contemplated, however, that the acidifying chamber and the ammoniation chamber can overlap to an extent so that both acid and ammonia will contact the material simultaneously during a portion of its passage through the drum.
The desired rapid and uniform contact between acid and particulate material in the acidifying chamber of the drum is accomplished by spraying the acid in finely atomized form onto a thin, free-falling stream or curtain of the particulate material. The stream or curtain is formed by internal lifting flights which lift the material such that it cascades downwardly upon sliding from the flights. The stationary acid spray nozzles are located at spaced intervals along the length of the chamber and are so arranged that the downwardly falling particles pass in front of the nozzle orifices. Thus acid is sprayed on free-flowing material and is not sprayed directly onto the wall of the chamber of directly onto the bulk material being carried by the flights. The manner in which the flights are canted insures good mixing without buildup or reverse flow problems. Preferably the flights are canted (relative to a plane passing through the axis of the drum) in a direction opposite the direction of rotation of the drum. A particularly suitable angle is about 45.degree.. The width (height) of the flights should generally between 10% and 20% of the diameter of the drum, causing them to effectively decrease the retention time of the material in the chamber with respect to the subsequent chambers because a lot of the space underneath the flights remains essentially empty during operation. By maintaining a highly active but thin bed, good mixing of the particulate material occurs, resulting in extremely uniform acidulation.
In the ammoniation chamber the particulate material is in the form of a tumbling bed in contact with the wall of the drum, inasmuch as there are no lifting flights in this chamber. The ammonia is introduced from stationary discharge orifices located within the bed near the lower end of the bed so that the ammonia has the maximum time to disperse into the bed before being exposed to the surface of the bed or being brought near the surface of the bed. With proper control of the input of ammonia this arrangement assures that there is little or no ammonia in the atmosphere in the ammoniation chamber. Preferably the points of injection of ammonia are spaced from the wall of the drum no more than about 1/3 of the depth of the bed.